Most wind turbines include a yaw bearing assembly to rotate a nacelle assembly with respect to a tower. Yaw bearing assemblies can include bearings that enable the nacelle assembly to rotate on the tower. The yaw bearing assemblies are driven by yaw motors and yaw braking systems that are continuously activated to maintain and control a yaw direction of the nacelle assembly.
Some yaw bearing assemblies include sliding bearing assemblies with active braking modules and passive braking modules coupled about a ring gear. A passive sliding track is positioned between a frame of a nacelle assembly and a ring gear. The active brake assembly includes a friction pad that contacts a friction pad on the ring gear. While such a brake assembly facilitates maintaining a yaw direction of the nacelle assembly, the yaw motors must overcome the combined effect of aerodynamic loads and friction forces between the friction pads. Some yaw brake assemblies include multiple sliding tracks, which may make replacing and/or servicing friction pads within the brake assembly difficult. And some wind turbines include a yaw bearing assembly and a separate yaw brake system, which add to the complexity of wind turbine. As wind turbine rotor diameters get larger, yaw moments are becoming large and difficult to brake and control. This requires many yaw motors and very large braking forces from friction devices.
In the majority of conventional wind turbines, the hydraulic brake calipers are dimensioned to resist approximately 20% of the maximum aerodynamic load torque. The rest is provided by the motors' electro-brakes. A problem with this type of solution is that the brake caliper linings do not maintain a constant friction coefficient over time. The friction coefficient may be affected due to wear, temperature, brake disc conditions and undesired contamination (oil or grease). If the friction coefficient increases it may cause a premature failure to the brake calipers themselves. On the other hand, if the friction coefficient decreases it may encumber the gearbox motors and in the worst case it may wear down the annular gear. Furthermore, brake discs require frequent maintenance which increases the operational cost of the wind turbine and the yaw brake may further require electro-brakes in the gearboxes to resist the over torques. This may cause unpredictable damages in the gearboxes.
As such, it would be desirable for a yaw bearing assembly to include a braking system that does not require the yaw motor to overcome forces between two friction surfaces and that can apply bi-directional braking forces to resist wind turbine yawing loads during braking and normal operation. Further, it is desirable to have an integrated yaw bearing and bi-directional brake system.